The AGP Aperture is like a pretend space in your computer where your pictures and videos can wait and get ready to be shown on your screen. Your computer needs to use this space because if it showed everything right away, your screen would be really slow and not work very well.
So, when your computer wants to show a picture or video, it puts it in the AGP Aperture and starts getting it ready. Then, when it's ready, it sends it to your screen so you can see it quickly and clearly. This is important because pictures and videos have a lot of information and need to be sent to your screen carefully and quickly.
Think of the AGP Aperture like a dressing room for your pictures and videos - they go there to get ready before they come out to show off in front of you!